Spray Foam Under Roof Sheathing

It s not coming from above the roof and it s not some new moisture source resulting from the spray foam.

Spray foam under roof sheathing. Open cell spray polyurethane foam ocspf and closed cell spray polyurethane foam ccspf insulation sprayed to the underside of roof sheathing is a popular strategy for increasing roof insulation levels in all climate zones. Increasingly building scientists are investigating why osb roof sheathing on many spray foam insulated roofs stays damp for months at a time. Underside of roof decks since the late 60s early 70s and to my knowledge i have not seen or heard of a rotted substrate under and conditions closed cell foam was applied. We have an insulation contractor in the area that does the expanding spray in place foam insulation.

Polyurethane or polystyrene spray foams can provide thorough coverage to roof rafters and attic walls in much less time than is needed for other materials. That s about 2 5 inches of closed cell spray foam assuming r 6 per inch. To comply with the building code the spray foam layer should have an r value of r 15. Lstiburek takes blame for bad language.

Unlike solid insulation products that require careful measurement cutting and fitting spray foam can cover even the most uneven surface instantly. Moisture generated indoors or that infiltrates into the home is responsible for the bulk of the moisture in an attic insulated with spray foam on the underside of the roof sheathing. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. Spray foam on underside of roof sheathing.

This construction approach was first used in the 1970s and it has been implemented much more widely since it was first included in the international code council s icc model building code in 2007 irc section 806 4. Install spray foam insulation along the underside of the roof deck to create an insulated unvented attic which can provide a conditioned space for hvac equipment that is located in the attic. Unvented roof strategies with spray polyurethane foam spf have been used since the mid 1990s to provide durable and efficient building enclosures. The formula in the building code is somewhat more conservative than dana s recommendation.

The anecdotes and data are enough to provide a few rules of thumb for designers and builders who want to install spray foam on the underside of roof sheathing. Take your pick as long as your building inspector approves. If anyone has please contact me with evidence that being said open cell is a great product as well however in your case you.